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-<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
-<HTML><HEAD>
-<TITLE>Apache API notes</TITLE>
-</HEAD>
-<!-- Background white, links blue (unvisited), navy (visited), red (active) -->
-<BODY
- BGCOLOR="#FFFFFF"
- TEXT="#000000"
- LINK="#0000FF"
- VLINK="#000080"
- ALINK="#FF0000"
->
-<!--#include virtual="header.html" -->
-
-<blockquote><strong>Warning:</strong>
-This document has not been updated to take into account changes
-made in the 2.0 version of the Apache HTTP Server. Some of the
-information may still be relevant, but please use it
-with care.
-</blockquote>
-
-<H1 ALIGN="CENTER">Apache API notes</H1>
-
-These are some notes on the Apache API and the data structures you
-have to deal with, <EM>etc.</EM> They are not yet nearly complete, but
-hopefully, they will help you get your bearings. Keep in mind that
-the API is still subject to change as we gain experience with it.
-(See the TODO file for what <EM>might</EM> be coming). However,
-it will be easy to adapt modules to any changes that are made.
-(We have more modules to adapt than you do).
-<P>
-
-A few notes on general pedagogical style here. In the interest of
-conciseness, all structure declarations here are incomplete --- the
-real ones have more slots that I'm not telling you about. For the
-most part, these are reserved to one component of the server core or
-another, and should be altered by modules with caution. However, in
-some cases, they really are things I just haven't gotten around to
-yet. Welcome to the bleeding edge.<P>
-
-Finally, here's an outline, to give you some bare idea of what's
-coming up, and in what order:
-
-<UL>
-<LI> <A HREF="#basics">Basic concepts.</A>
-<MENU>
- <LI> <A HREF="#HMR">Handlers, Modules, and Requests</A>
- <LI> <A HREF="#moduletour">A brief tour of a module</A>
-</MENU>
-<LI> <A HREF="#handlers">How handlers work</A>
-<MENU>
- <LI> <A HREF="#req_tour">A brief tour of the <CODE>request_rec</CODE></A>
- <LI> <A HREF="#req_orig">Where request_rec structures come from</A>
- <LI> <A HREF="#req_return">Handling requests, declining, and returning error
- codes</A>
- <LI> <A HREF="#resp_handlers">Special considerations for response handlers</A>
- <LI> <A HREF="#auth_handlers">Special considerations for authentication
- handlers</A>
- <LI> <A HREF="#log_handlers">Special considerations for logging handlers</A>
-</MENU>
-<LI> <A HREF="#pools">Resource allocation and resource pools</A>
-<LI> <A HREF="#config">Configuration, commands and the like</A>
-<MENU>
- <LI> <A HREF="#per-dir">Per-directory configuration structures</A>
- <LI> <A HREF="#commands">Command handling</A>
- <LI> <A HREF="#servconf">Side notes --- per-server configuration,
- virtual servers, <EM>etc</EM>.</A>
-</MENU>
-</UL>
-
-<H2><A NAME="basics">Basic concepts.</A></H2>
-
-We begin with an overview of the basic concepts behind the
-API, and how they are manifested in the code.
-
-<H3><A NAME="HMR">Handlers, Modules, and Requests</A></H3>
-
-Apache breaks down request handling into a series of steps, more or
-less the same way the Netscape server API does (although this API has
-a few more stages than NetSite does, as hooks for stuff I thought
-might be useful in the future). These are:
-
-<UL>
- <LI> URI -&gt; Filename translation
- <LI> Auth ID checking [is the user who they say they are?]
- <LI> Auth access checking [is the user authorized <EM>here</EM>?]
- <LI> Access checking other than auth
- <LI> Determining MIME type of the object requested
- <LI> `Fixups' --- there aren't any of these yet, but the phase is
- intended as a hook for possible extensions like
- <CODE>SetEnv</CODE>, which don't really fit well elsewhere.
- <LI> Actually sending a response back to the client.
- <LI> Logging the request
-</UL>
-
-These phases are handled by looking at each of a succession of
-<EM>modules</EM>, looking to see if each of them has a handler for the
-phase, and attempting invoking it if so. The handler can typically do
-one of three things:
-
-<UL>
- <LI> <EM>Handle</EM> the request, and indicate that it has done so
- by returning the magic constant <CODE>OK</CODE>.
- <LI> <EM>Decline</EM> to handle the request, by returning the magic
- integer constant <CODE>DECLINED</CODE>. In this case, the
- server behaves in all respects as if the handler simply hadn't
- been there.
- <LI> Signal an error, by returning one of the HTTP error codes.
- This terminates normal handling of the request, although an
- ErrorDocument may be invoked to try to mop up, and it will be
- logged in any case.
-</UL>
-
-Most phases are terminated by the first module that handles them;
-however, for logging, `fixups', and non-access authentication
-checking, all handlers always run (barring an error). Also, the
-response phase is unique in that modules may declare multiple handlers
-for it, via a dispatch table keyed on the MIME type of the requested
-object. Modules may declare a response-phase handler which can handle
-<EM>any</EM> request, by giving it the key <CODE>*/*</CODE> (<EM>i.e.</EM>, a
-wildcard MIME type specification). However, wildcard handlers are
-only invoked if the server has already tried and failed to find a more
-specific response handler for the MIME type of the requested object
-(either none existed, or they all declined).<P>
-
-The handlers themselves are functions of one argument (a
-<CODE>request_rec</CODE> structure. vide infra), which returns an
-integer, as above.<P>
-
-<H3><A NAME="moduletour">A brief tour of a module</A></H3>
-
-At this point, we need to explain the structure of a module. Our
-candidate will be one of the messier ones, the CGI module --- this
-handles both CGI scripts and the <CODE>ScriptAlias</CODE> config file
-command. It's actually a great deal more complicated than most
-modules, but if we're going to have only one example, it might as well
-be the one with its fingers in every place.<P>
-
-Let's begin with handlers. In order to handle the CGI scripts, the
-module declares a response handler for them. Because of
-<CODE>ScriptAlias</CODE>, it also has handlers for the name
-translation phase (to recognize <CODE>ScriptAlias</CODE>ed URIs), the
-type-checking phase (any <CODE>ScriptAlias</CODE>ed request is typed
-as a CGI script).<P>
-
-The module needs to maintain some per (virtual)
-server information, namely, the <CODE>ScriptAlias</CODE>es in effect;
-the module structure therefore contains pointers to a functions which
-builds these structures, and to another which combines two of them (in
-case the main server and a virtual server both have
-<CODE>ScriptAlias</CODE>es declared).<P>
-
-Finally, this module contains code to handle the
-<CODE>ScriptAlias</CODE> command itself. This particular module only
-declares one command, but there could be more, so modules have
-<EM>command tables</EM> which declare their commands, and describe
-where they are permitted, and how they are to be invoked. <P>
-
-A final note on the declared types of the arguments of some of these
-commands: a <CODE>pool</CODE> is a pointer to a <EM>resource pool</EM>
-structure; these are used by the server to keep track of the memory
-which has been allocated, files opened, <EM>etc.</EM>, either to service a
-particular request, or to handle the process of configuring itself.
-That way, when the request is over (or, for the configuration pool,
-when the server is restarting), the memory can be freed, and the files
-closed, <EM>en masse</EM>, without anyone having to write explicit code to
-track them all down and dispose of them. Also, a
-<CODE>cmd_parms</CODE> structure contains various information about
-the config file being read, and other status information, which is
-sometimes of use to the function which processes a config-file command
-(such as <CODE>ScriptAlias</CODE>).
-
-With no further ado, the module itself:
-
-<PRE>
-/* Declarations of handlers. */
-
-int translate_scriptalias (request_rec *);
-int type_scriptalias (request_rec *);
-int cgi_handler (request_rec *);
-
-/* Subsidiary dispatch table for response-phase handlers, by MIME type */
-
-handler_rec cgi_handlers[] = {
-{ "application/x-httpd-cgi", cgi_handler },
-{ NULL }
-};
-
-/* Declarations of routines to manipulate the module's configuration
- * info. Note that these are returned, and passed in, as void *'s;
- * the server core keeps track of them, but it doesn't, and can't,
- * know their internal structure.
- */
-
-void *make_cgi_server_config (pool *);
-void *merge_cgi_server_config (pool *, void *, void *);
-
-/* Declarations of routines to handle config-file commands */
-
-extern char *script_alias(cmd_parms *, void *per_dir_config, char *fake,
- char *real);
-
-command_rec cgi_cmds[] = {
-{ "ScriptAlias", script_alias, NULL, RSRC_CONF, TAKE2,
- "a fakename and a realname"},
-{ NULL }
-};
-
-module cgi_module = {
- STANDARD_MODULE_STUFF,
- NULL, /* initializer */
- NULL, /* dir config creator */
- NULL, /* dir merger --- default is to override */
- make_cgi_server_config, /* server config */
- merge_cgi_server_config, /* merge server config */
- cgi_cmds, /* command table */
- cgi_handlers, /* handlers */
- translate_scriptalias, /* filename translation */
- NULL, /* check_user_id */
- NULL, /* check auth */
- NULL, /* check access */
- type_scriptalias, /* type_checker */
- NULL, /* fixups */
- NULL, /* logger */
- NULL /* header parser */
-};
-</PRE>
-
-<H2><A NAME="handlers">How handlers work</A></H2>
-
-The sole argument to handlers is a <CODE>request_rec</CODE> structure.
-This structure describes a particular request which has been made to
-the server, on behalf of a client. In most cases, each connection to
-the client generates only one <CODE>request_rec</CODE> structure.<P>
-
-<H3><A NAME="req_tour">A brief tour of the <CODE>request_rec</CODE></A></H3>
-
-The <CODE>request_rec</CODE> contains pointers to a resource pool
-which will be cleared when the server is finished handling the
-request; to structures containing per-server and per-connection
-information, and most importantly, information on the request itself.<P>
-
-The most important such information is a small set of character
-strings describing attributes of the object being requested, including
-its URI, filename, content-type and content-encoding (these being filled
-in by the translation and type-check handlers which handle the
-request, respectively). <P>
-
-Other commonly used data items are tables giving the MIME headers on
-the client's original request, MIME headers to be sent back with the
-response (which modules can add to at will), and environment variables
-for any subprocesses which are spawned off in the course of servicing
-the request. These tables are manipulated using the
-<CODE>ap_table_get</CODE> and <CODE>ap_table_set</CODE> routines. <P>
-<BLOCKQUOTE>
- Note that the <SAMP>Content-type</SAMP> header value <EM>cannot</EM> be
- set by module content-handlers using the <SAMP>ap_table_*()</SAMP>
- routines. Rather, it is set by pointing the <SAMP>content_type</SAMP>
- field in the <SAMP>request_rec</SAMP> structure to an appropriate
- string. <EM>E.g.</EM>,
- <PRE>
- r-&gt;content_type = "text/html";
- </PRE>
-</BLOCKQUOTE>
-Finally, there are pointers to two data structures which, in turn,
-point to per-module configuration structures. Specifically, these
-hold pointers to the data structures which the module has built to
-describe the way it has been configured to operate in a given
-directory (via <CODE>.htaccess</CODE> files or
-<CODE>&lt;Directory&gt;</CODE> sections), for private data it has
-built in the course of servicing the request (so modules' handlers for
-one phase can pass `notes' to their handlers for other phases). There
-is another such configuration vector in the <CODE>server_rec</CODE>
-data structure pointed to by the <CODE>request_rec</CODE>, which
-contains per (virtual) server configuration data.<P>
-
-Here is an abridged declaration, giving the fields most commonly used:<P>
-
-<PRE>
-struct request_rec {
-
- pool *pool;
- conn_rec *connection;
- server_rec *server;
-
- /* What object is being requested */
-
- char *uri;
- char *filename;
- char *path_info;
- char *args; /* QUERY_ARGS, if any */
- struct stat finfo; /* Set by server core;
- * st_mode set to zero if no such file */
-
- char *content_type;
- char *content_encoding;
-
- /* MIME header environments, in and out. Also, an array containing
- * environment variables to be passed to subprocesses, so people can
- * write modules to add to that environment.
- *
- * The difference between headers_out and err_headers_out is that
- * the latter are printed even on error, and persist across internal
- * redirects (so the headers printed for ErrorDocument handlers will
- * have them).
- */
-
- table *headers_in;
- table *headers_out;
- table *err_headers_out;
- table *subprocess_env;
-
- /* Info about the request itself... */
-
- int header_only; /* HEAD request, as opposed to GET */
- char *protocol; /* Protocol, as given to us, or HTTP/0.9 */
- char *method; /* GET, HEAD, POST, <EM>etc.</EM> */
- int method_number; /* M_GET, M_POST, <EM>etc.</EM> */
-
- /* Info for logging */
-
- char *the_request;
- int bytes_sent;
-
- /* A flag which modules can set, to indicate that the data being
- * returned is volatile, and clients should be told not to cache it.
- */
-
- int no_cache;
-
- /* Various other config info which may change with .htaccess files
- * These are config vectors, with one void* pointer for each module
- * (the thing pointed to being the module's business).
- */
-
- void *per_dir_config; /* Options set in config files, <EM>etc.</EM> */
- void *request_config; /* Notes on *this* request */
-
-};
-
-</PRE>
-
-<H3><A NAME="req_orig">Where request_rec structures come from</A></H3>
-
-Most <CODE>request_rec</CODE> structures are built by reading an HTTP
-request from a client, and filling in the fields. However, there are
-a few exceptions:
-
-<UL>
- <LI> If the request is to an imagemap, a type map (<EM>i.e.</EM>, a
- <CODE>*.var</CODE> file), or a CGI script which returned a
- local `Location:', then the resource which the user requested
- is going to be ultimately located by some URI other than what
- the client originally supplied. In this case, the server does
- an <EM>internal redirect</EM>, constructing a new
- <CODE>request_rec</CODE> for the new URI, and processing it
- almost exactly as if the client had requested the new URI
- directly. <P>
-
- <LI> If some handler signaled an error, and an
- <CODE>ErrorDocument</CODE> is in scope, the same internal
- redirect machinery comes into play.<P>
-
- <LI> Finally, a handler occasionally needs to investigate `what
- would happen if' some other request were run. For instance,
- the directory indexing module needs to know what MIME type
- would be assigned to a request for each directory entry, in
- order to figure out what icon to use.<P>
-
- Such handlers can construct a <EM>sub-request</EM>, using the
- functions <CODE>ap_sub_req_lookup_file</CODE>,
- <CODE>ap_sub_req_lookup_uri</CODE>, and
- <CODE>ap_sub_req_method_uri</CODE>; these construct a new
- <CODE>request_rec</CODE> structure and processes it as you
- would expect, up to but not including the point of actually
- sending a response. (These functions skip over the access
- checks if the sub-request is for a file in the same directory
- as the original request).<P>
-
- (Server-side includes work by building sub-requests and then
- actually invoking the response handler for them, via the
- function <CODE>ap_run_sub_req</CODE>).
-</UL>
-
-<H3><A NAME="req_return">Handling requests, declining, and returning error
- codes</A></H3>
-
-As discussed above, each handler, when invoked to handle a particular
-<CODE>request_rec</CODE>, has to return an <CODE>int</CODE> to
-indicate what happened. That can either be
-
-<UL>
- <LI> OK --- the request was handled successfully. This may or may
- not terminate the phase.
- <LI> DECLINED --- no erroneous condition exists, but the module
- declines to handle the phase; the server tries to find another.
- <LI> an HTTP error code, which aborts handling of the request.
-</UL>
-
-Note that if the error code returned is <CODE>REDIRECT</CODE>, then
-the module should put a <CODE>Location</CODE> in the request's
-<CODE>headers_out</CODE>, to indicate where the client should be
-redirected <EM>to</EM>. <P>
-
-<H3><A NAME="resp_handlers">Special considerations for response
- handlers</A></H3>
-
-Handlers for most phases do their work by simply setting a few fields
-in the <CODE>request_rec</CODE> structure (or, in the case of access
-checkers, simply by returning the correct error code). However,
-response handlers have to actually send a request back to the client. <P>
-
-They should begin by sending an HTTP response header, using the
-function <CODE>ap_send_http_header</CODE>. (You don't have to do
-anything special to skip sending the header for HTTP/0.9 requests; the
-function figures out on its own that it shouldn't do anything). If
-the request is marked <CODE>header_only</CODE>, that's all they should
-do; they should return after that, without attempting any further
-output. <P>
-
-Otherwise, they should produce a request body which responds to the
-client as appropriate. The primitives for this are <CODE>ap_rputc</CODE>
-and <CODE>ap_rprintf</CODE>, for internally generated output, and
-<CODE>ap_send_fd</CODE>, to copy the contents of some <CODE>FILE *</CODE>
-straight to the client. <P>
-
-At this point, you should more or less understand the following piece
-of code, which is the handler which handles <CODE>GET</CODE> requests
-which have no more specific handler; it also shows how conditional
-<CODE>GET</CODE>s can be handled, if it's desirable to do so in a
-particular response handler --- <CODE>ap_set_last_modified</CODE> checks
-against the <CODE>If-modified-since</CODE> value supplied by the
-client, if any, and returns an appropriate code (which will, if
-nonzero, be USE_LOCAL_COPY). No similar considerations apply for
-<CODE>ap_set_content_length</CODE>, but it returns an error code for
-symmetry.<P>
-
-<PRE>
-int default_handler (request_rec *r)
-{
- int errstatus;
- FILE *f;
-
- if (r-&gt;method_number != M_GET) return DECLINED;
- if (r-&gt;finfo.st_mode == 0) return NOT_FOUND;
-
- if ((errstatus = ap_set_content_length (r, r-&gt;finfo.st_size))
- || (errstatus = ap_set_last_modified (r, r-&gt;finfo.st_mtime)))
- return errstatus;
-
- f = fopen (r-&gt;filename, "r");
-
- if (f == NULL) {
- log_reason("file permissions deny server access",
- r-&gt;filename, r);
- return FORBIDDEN;
- }
-
- register_timeout ("send", r);
- ap_send_http_header (r);
-
- if (!r-&gt;header_only) send_fd (f, r);
- ap_pfclose (r-&gt;pool, f);
- return OK;
-}
-</PRE>
-
-Finally, if all of this is too much of a challenge, there are a few
-ways out of it. First off, as shown above, a response handler which
-has not yet produced any output can simply return an error code, in
-which case the server will automatically produce an error response.
-Secondly, it can punt to some other handler by invoking
-<CODE>ap_internal_redirect</CODE>, which is how the internal redirection
-machinery discussed above is invoked. A response handler which has
-internally redirected should always return <CODE>OK</CODE>. <P>
-
-(Invoking <CODE>ap_internal_redirect</CODE> from handlers which are
-<EM>not</EM> response handlers will lead to serious confusion).
-
-<H3><A NAME="auth_handlers">Special considerations for authentication
- handlers</A></H3>
-
-Stuff that should be discussed here in detail:
-
-<UL>
- <LI> Authentication-phase handlers not invoked unless auth is
- configured for the directory.
- <LI> Common auth configuration stored in the core per-dir
- configuration; it has accessors <CODE>ap_auth_type</CODE>,
- <CODE>ap_auth_name</CODE>, and <CODE>ap_requires</CODE>.
- <LI> Common routines, to handle the protocol end of things, at least
- for HTTP basic authentication (<CODE>ap_get_basic_auth_pw</CODE>,
- which sets the <CODE>connection-&gt;user</CODE> structure field
- automatically, and <CODE>ap_note_basic_auth_failure</CODE>, which
- arranges for the proper <CODE>WWW-Authenticate:</CODE> header
- to be sent back).
-</UL>
-
-<H3><A NAME="log_handlers">Special considerations for logging handlers</A></H3>
-
-When a request has internally redirected, there is the question of
-what to log. Apache handles this by bundling the entire chain of
-redirects into a list of <CODE>request_rec</CODE> structures which are
-threaded through the <CODE>r-&gt;prev</CODE> and <CODE>r-&gt;next</CODE>
-pointers. The <CODE>request_rec</CODE> which is passed to the logging
-handlers in such cases is the one which was originally built for the
-initial request from the client; note that the bytes_sent field will
-only be correct in the last request in the chain (the one for which a
-response was actually sent).
-
-<H2><A NAME="pools">Resource allocation and resource pools</A></H2>
-<P>
-One of the problems of writing and designing a server-pool server is
-that of preventing leakage, that is, allocating resources (memory,
-open files, <EM>etc.</EM>), without subsequently releasing them. The resource
-pool machinery is designed to make it easy to prevent this from
-happening, by allowing resource to be allocated in such a way that
-they are <EM>automatically</EM> released when the server is done with
-them.
-</P>
-<P>
-The way this works is as follows: the memory which is allocated, file
-opened, <EM>etc.</EM>, to deal with a particular request are tied to a
-<EM>resource pool</EM> which is allocated for the request. The pool
-is a data structure which itself tracks the resources in question.
-</P>
-<P>
-When the request has been processed, the pool is <EM>cleared</EM>. At
-that point, all the memory associated with it is released for reuse,
-all files associated with it are closed, and any other clean-up
-functions which are associated with the pool are run. When this is
-over, we can be confident that all the resource tied to the pool have
-been released, and that none of them have leaked.
-</P>
-<P>
-Server restarts, and allocation of memory and resources for per-server
-configuration, are handled in a similar way. There is a
-<EM>configuration pool</EM>, which keeps track of resources which were
-allocated while reading the server configuration files, and handling
-the commands therein (for instance, the memory that was allocated for
-per-server module configuration, log files and other files that were
-opened, and so forth). When the server restarts, and has to reread
-the configuration files, the configuration pool is cleared, and so the
-memory and file descriptors which were taken up by reading them the
-last time are made available for reuse.
-</P>
-<P>
-It should be noted that use of the pool machinery isn't generally
-obligatory, except for situations like logging handlers, where you
-really need to register cleanups to make sure that the log file gets
-closed when the server restarts (this is most easily done by using the
-function <CODE><A HREF="#pool-files">ap_pfopen</A></CODE>, which also
-arranges for the underlying file descriptor to be closed before any
-child processes, such as for CGI scripts, are <CODE>exec</CODE>ed), or
-in case you are using the timeout machinery (which isn't yet even
-documented here). However, there are two benefits to using it:
-resources allocated to a pool never leak (even if you allocate a
-scratch string, and just forget about it); also, for memory
-allocation, <CODE>ap_palloc</CODE> is generally faster than
-<CODE>malloc</CODE>.
-</P>
-<P>
-We begin here by describing how memory is allocated to pools, and then
-discuss how other resources are tracked by the resource pool
-machinery.
-</P>
-<H3>Allocation of memory in pools</H3>
-<P>
-Memory is allocated to pools by calling the function
-<CODE>ap_palloc</CODE>, which takes two arguments, one being a pointer to
-a resource pool structure, and the other being the amount of memory to
-allocate (in <CODE>char</CODE>s). Within handlers for handling
-requests, the most common way of getting a resource pool structure is
-by looking at the <CODE>pool</CODE> slot of the relevant
-<CODE>request_rec</CODE>; hence the repeated appearance of the
-following idiom in module code:
-</P>
-<PRE>
-int my_handler(request_rec *r)
-{
- struct my_structure *foo;
- ...
-
- foo = (foo *)ap_palloc (r-&gt;pool, sizeof(my_structure));
-}
-</PRE>
-<P>
-Note that <EM>there is no <CODE>ap_pfree</CODE></EM> ---
-<CODE>ap_palloc</CODE>ed memory is freed only when the associated
-resource pool is cleared. This means that <CODE>ap_palloc</CODE> does not
-have to do as much accounting as <CODE>malloc()</CODE>; all it does in
-the typical case is to round up the size, bump a pointer, and do a
-range check.
-</P>
-<P>
-(It also raises the possibility that heavy use of <CODE>ap_palloc</CODE>
-could cause a server process to grow excessively large. There are
-two ways to deal with this, which are dealt with below; briefly, you
-can use <CODE>malloc</CODE>, and try to be sure that all of the memory
-gets explicitly <CODE>free</CODE>d, or you can allocate a sub-pool of
-the main pool, allocate your memory in the sub-pool, and clear it out
-periodically. The latter technique is discussed in the section on
-sub-pools below, and is used in the directory-indexing code, in order
-to avoid excessive storage allocation when listing directories with
-thousands of files).
-</P>
-<H3>Allocating initialized memory</H3>
-<P>
-There are functions which allocate initialized memory, and are
-frequently useful. The function <CODE>ap_pcalloc</CODE> has the same
-interface as <CODE>ap_palloc</CODE>, but clears out the memory it
-allocates before it returns it. The function <CODE>ap_pstrdup</CODE>
-takes a resource pool and a <CODE>char *</CODE> as arguments, and
-allocates memory for a copy of the string the pointer points to,
-returning a pointer to the copy. Finally <CODE>ap_pstrcat</CODE> is a
-varargs-style function, which takes a pointer to a resource pool, and
-at least two <CODE>char *</CODE> arguments, the last of which must be
-<CODE>NULL</CODE>. It allocates enough memory to fit copies of each
-of the strings, as a unit; for instance:
-</P>
-<PRE>
- ap_pstrcat (r-&gt;pool, "foo", "/", "bar", NULL);
-</PRE>
-<P>
-returns a pointer to 8 bytes worth of memory, initialized to
-<CODE>"foo/bar"</CODE>.
-</P>
-<H3><A NAME="pools-used">Commonly-used pools in the Apache Web server</A></H3>
-<P>
-A pool is really defined by its lifetime more than anything else. There
-are some static pools in http_main which are passed to various
-non-http_main functions as arguments at opportune times. Here they are:
-</P>
-<DL COMPACT>
- <DT>permanent_pool
- </DT>
- <DD>
- <UL>
- <LI>never passed to anything else, this is the ancestor of all pools
- </LI>
- </UL>
- </DD>
- <DT>pconf
- </DT>
- <DD>
- <UL>
- <LI>subpool of permanent_pool
- </LI>
- <LI>created at the beginning of a config "cycle"; exists until the
- server is terminated or restarts; passed to all config-time
- routines, either via cmd-&gt;pool, or as the "pool *p" argument on
- those which don't take pools
- </LI>
- <LI>passed to the module init() functions
- </LI>
- </UL>
- </DD>
- <DT>ptemp
- </DT>
- <DD>
- <UL>
- <LI>sorry I lie, this pool isn't called this currently in 1.3, I
- renamed it this in my pthreads development. I'm referring to
- the use of ptrans in the parent... contrast this with the later
- definition of ptrans in the child.
- </LI>
- <LI>subpool of permanent_pool
- </LI>
- <LI>created at the beginning of a config "cycle"; exists until the
- end of config parsing; passed to config-time routines <EM>via</EM>
- cmd-&gt;temp_pool. Somewhat of a "bastard child" because it isn't
- available everywhere. Used for temporary scratch space which
- may be needed by some config routines but which is deleted at
- the end of config.
- </LI>
- </UL>
- </DD>
- <DT>pchild
- </DT>
- <DD>
- <UL>
- <LI>subpool of permanent_pool
- </LI>
- <LI>created when a child is spawned (or a thread is created); lives
- until that child (thread) is destroyed
- </LI>
- <LI>passed to the module child_init functions
- </LI>
- <LI>destruction happens right after the child_exit functions are
- called... (which may explain why I think child_exit is redundant
- and unneeded)
- </LI>
- </UL>
- </DD>
- <DT>ptrans
- <DT>
- <DD>
- <UL>
- <LI>should be a subpool of pchild, but currently is a subpool of
- permanent_pool, see above
- </LI>
- <LI>cleared by the child before going into the accept() loop to receive
- a connection
- </LI>
- <LI>used as connection-&gt;pool
- </LI>
- </UL>
- </DD>
- <DT>r-&gt;pool
- </DT>
- <DD>
- <UL>
- <LI>for the main request this is a subpool of connection-&gt;pool; for
- subrequests it is a subpool of the parent request's pool.
- </LI>
- <LI>exists until the end of the request (<EM>i.e.</EM>,
- ap_destroy_sub_req, or
- in child_main after process_request has finished)
- </LI>
- <LI>note that r itself is allocated from r-&gt;pool; <EM>i.e.</EM>,
- r-&gt;pool is
- first created and then r is the first thing palloc()d from it
- </LI>
- </UL>
- </DD>
-</DL>
-<P>
-For almost everything folks do, r-&gt;pool is the pool to use. But you
-can see how other lifetimes, such as pchild, are useful to some
-modules... such as modules that need to open a database connection once
-per child, and wish to clean it up when the child dies.
-</P>
-<P>
-You can also see how some bugs have manifested themself, such as setting
-connection-&gt;user to a value from r-&gt;pool -- in this case
-connection exists
-for the lifetime of ptrans, which is longer than r-&gt;pool (especially if
-r-&gt;pool is a subrequest!). So the correct thing to do is to allocate
-from connection-&gt;pool.
-</P>
-<P>
-And there was another interesting bug in mod_include/mod_cgi. You'll see
-in those that they do this test to decide if they should use r-&gt;pool
-or r-&gt;main-&gt;pool. In this case the resource that they are registering
-for cleanup is a child process. If it were registered in r-&gt;pool,
-then the code would wait() for the child when the subrequest finishes.
-With mod_include this could be any old #include, and the delay can be up
-to 3 seconds... and happened quite frequently. Instead the subprocess
-is registered in r-&gt;main-&gt;pool which causes it to be cleaned up when
-the entire request is done -- <EM>i.e.</EM>, after the output has been sent to
-the client and logging has happened.
-</P>
-<H3><A NAME="pool-files">Tracking open files, etc.</A></H3>
-<P>
-As indicated above, resource pools are also used to track other sorts
-of resources besides memory. The most common are open files. The
-routine which is typically used for this is <CODE>ap_pfopen</CODE>, which
-takes a resource pool and two strings as arguments; the strings are
-the same as the typical arguments to <CODE>fopen</CODE>, <EM>e.g.</EM>,
-</P>
-<PRE>
- ...
- FILE *f = ap_pfopen (r-&gt;pool, r-&gt;filename, "r");
-
- if (f == NULL) { ... } else { ... }
-</PRE>
-<P>
-There is also a <CODE>ap_popenf</CODE> routine, which parallels the
-lower-level <CODE>open</CODE> system call. Both of these routines
-arrange for the file to be closed when the resource pool in question
-is cleared.
-</P>
-<P>
-Unlike the case for memory, there <EM>are</EM> functions to close
-files allocated with <CODE>ap_pfopen</CODE>, and <CODE>ap_popenf</CODE>,
-namely <CODE>ap_pfclose</CODE> and <CODE>ap_pclosef</CODE>. (This is
-because, on many systems, the number of files which a single process
-can have open is quite limited). It is important to use these
-functions to close files allocated with <CODE>ap_pfopen</CODE> and
-<CODE>ap_popenf</CODE>, since to do otherwise could cause fatal errors on
-systems such as Linux, which react badly if the same
-<CODE>FILE*</CODE> is closed more than once.
-</P>
-<P>
-(Using the <CODE>close</CODE> functions is not mandatory, since the
-file will eventually be closed regardless, but you should consider it
-in cases where your module is opening, or could open, a lot of files).
-</P>
-<H3>Other sorts of resources --- cleanup functions</H3>
-<BLOCKQUOTE>
-More text goes here. Describe the the cleanup primitives in terms of
-which the file stuff is implemented; also, <CODE>spawn_process</CODE>.
-</BLOCKQUOTE>
-<P>
-Pool cleanups live until clear_pool() is called: clear_pool(a) recursively
-calls destroy_pool() on all subpools of a; then calls all the cleanups for a;
-then releases all the memory for a. destroy_pool(a) calls clear_pool(a)
-and then releases the pool structure itself. <EM>i.e.</EM>, clear_pool(a) doesn't
-delete a, it just frees up all the resources and you can start using it
-again immediately.
-</P>
-<H3>Fine control --- creating and dealing with sub-pools, with a note
-on sub-requests</H3>
-
-On rare occasions, too-free use of <CODE>ap_palloc()</CODE> and the
-associated primitives may result in undesirably profligate resource
-allocation. You can deal with such a case by creating a
-<EM>sub-pool</EM>, allocating within the sub-pool rather than the main
-pool, and clearing or destroying the sub-pool, which releases the
-resources which were associated with it. (This really <EM>is</EM> a
-rare situation; the only case in which it comes up in the standard
-module set is in case of listing directories, and then only with
-<EM>very</EM> large directories. Unnecessary use of the primitives
-discussed here can hair up your code quite a bit, with very little
-gain). <P>
-
-The primitive for creating a sub-pool is <CODE>ap_make_sub_pool</CODE>,
-which takes another pool (the parent pool) as an argument. When the
-main pool is cleared, the sub-pool will be destroyed. The sub-pool
-may also be cleared or destroyed at any time, by calling the functions
-<CODE>ap_clear_pool</CODE> and <CODE>ap_destroy_pool</CODE>, respectively.
-(The difference is that <CODE>ap_clear_pool</CODE> frees resources
-associated with the pool, while <CODE>ap_destroy_pool</CODE> also
-deallocates the pool itself. In the former case, you can allocate new
-resources within the pool, and clear it again, and so forth; in the
-latter case, it is simply gone). <P>
-
-One final note --- sub-requests have their own resource pools, which
-are sub-pools of the resource pool for the main request. The polite
-way to reclaim the resources associated with a sub request which you
-have allocated (using the <CODE>ap_sub_req_...</CODE> functions)
-is <CODE>ap_destroy_sub_req</CODE>, which frees the resource pool.
-Before calling this function, be sure to copy anything that you care
-about which might be allocated in the sub-request's resource pool into
-someplace a little less volatile (for instance, the filename in its
-<CODE>request_rec</CODE> structure). <P>
-
-(Again, under most circumstances, you shouldn't feel obliged to call
-this function; only 2K of memory or so are allocated for a typical sub
-request, and it will be freed anyway when the main request pool is
-cleared. It is only when you are allocating many, many sub-requests
-for a single main request that you should seriously consider the
-<CODE>ap_destroy_...</CODE> functions).
-
-<H2><A NAME="config">Configuration, commands and the like</A></H2>
-
-One of the design goals for this server was to maintain external
-compatibility with the NCSA 1.3 server --- that is, to read the same
-configuration files, to process all the directives therein correctly,
-and in general to be a drop-in replacement for NCSA. On the other
-hand, another design goal was to move as much of the server's
-functionality into modules which have as little as possible to do with
-the monolithic server core. The only way to reconcile these goals is
-to move the handling of most commands from the central server into the
-modules. <P>
-
-However, just giving the modules command tables is not enough to
-divorce them completely from the server core. The server has to
-remember the commands in order to act on them later. That involves
-maintaining data which is private to the modules, and which can be
-either per-server, or per-directory. Most things are per-directory,
-including in particular access control and authorization information,
-but also information on how to determine file types from suffixes,
-which can be modified by <CODE>AddType</CODE> and
-<CODE>DefaultType</CODE> directives, and so forth. In general, the
-governing philosophy is that anything which <EM>can</EM> be made
-configurable by directory should be; per-server information is
-generally used in the standard set of modules for information like
-<CODE>Alias</CODE>es and <CODE>Redirect</CODE>s which come into play
-before the request is tied to a particular place in the underlying
-file system. <P>
-
-Another requirement for emulating the NCSA server is being able to
-handle the per-directory configuration files, generally called
-<CODE>.htaccess</CODE> files, though even in the NCSA server they can
-contain directives which have nothing at all to do with access
-control. Accordingly, after URI -&gt; filename translation, but before
-performing any other phase, the server walks down the directory
-hierarchy of the underlying filesystem, following the translated
-pathname, to read any <CODE>.htaccess</CODE> files which might be
-present. The information which is read in then has to be
-<EM>merged</EM> with the applicable information from the server's own
-config files (either from the <CODE>&lt;Directory&gt;</CODE> sections
-in <CODE>access.conf</CODE>, or from defaults in
-<CODE>srm.conf</CODE>, which actually behaves for most purposes almost
-exactly like <CODE>&lt;Directory /&gt;</CODE>).<P>
-
-Finally, after having served a request which involved reading
-<CODE>.htaccess</CODE> files, we need to discard the storage allocated
-for handling them. That is solved the same way it is solved wherever
-else similar problems come up, by tying those structures to the
-per-transaction resource pool. <P>
-
-<H3><A NAME="per-dir">Per-directory configuration structures</A></H3>
-
-Let's look out how all of this plays out in <CODE>mod_mime.c</CODE>,
-which defines the file typing handler which emulates the NCSA server's
-behavior of determining file types from suffixes. What we'll be
-looking at, here, is the code which implements the
-<CODE>AddType</CODE> and <CODE>AddEncoding</CODE> commands. These
-commands can appear in <CODE>.htaccess</CODE> files, so they must be
-handled in the module's private per-directory data, which in fact,
-consists of two separate <CODE>table</CODE>s for MIME types and
-encoding information, and is declared as follows:
-
-<PRE>
-typedef struct {
- table *forced_types; /* Additional AddTyped stuff */
- table *encoding_types; /* Added with AddEncoding... */
-} mime_dir_config;
-</PRE>
-
-When the server is reading a configuration file, or
-<CODE>&lt;Directory&gt;</CODE> section, which includes one of the MIME
-module's commands, it needs to create a <CODE>mime_dir_config</CODE>
-structure, so those commands have something to act on. It does this
-by invoking the function it finds in the module's `create per-dir
-config slot', with two arguments: the name of the directory to which
-this configuration information applies (or <CODE>NULL</CODE> for
-<CODE>srm.conf</CODE>), and a pointer to a resource pool in which the
-allocation should happen. <P>
-
-(If we are reading a <CODE>.htaccess</CODE> file, that resource pool
-is the per-request resource pool for the request; otherwise it is a
-resource pool which is used for configuration data, and cleared on
-restarts. Either way, it is important for the structure being created
-to vanish when the pool is cleared, by registering a cleanup on the
-pool if necessary). <P>
-
-For the MIME module, the per-dir config creation function just
-<CODE>ap_palloc</CODE>s the structure above, and a creates a couple of
-<CODE>table</CODE>s to fill it. That looks like this:
-
-<PRE>
-void *create_mime_dir_config (pool *p, char *dummy)
-{
- mime_dir_config *new =
- (mime_dir_config *) ap_palloc (p, sizeof(mime_dir_config));
-
- new-&gt;forced_types = ap_make_table (p, 4);
- new-&gt;encoding_types = ap_make_table (p, 4);
-
- return new;
-}
-</PRE>
-
-Now, suppose we've just read in a <CODE>.htaccess</CODE> file. We
-already have the per-directory configuration structure for the next
-directory up in the hierarchy. If the <CODE>.htaccess</CODE> file we
-just read in didn't have any <CODE>AddType</CODE> or
-<CODE>AddEncoding</CODE> commands, its per-directory config structure
-for the MIME module is still valid, and we can just use it.
-Otherwise, we need to merge the two structures somehow. <P>
-
-To do that, the server invokes the module's per-directory config merge
-function, if one is present. That function takes three arguments:
-the two structures being merged, and a resource pool in which to
-allocate the result. For the MIME module, all that needs to be done
-is overlay the tables from the new per-directory config structure with
-those from the parent:
-
-<PRE>
-void *merge_mime_dir_configs (pool *p, void *parent_dirv, void *subdirv)
-{
- mime_dir_config *parent_dir = (mime_dir_config *)parent_dirv;
- mime_dir_config *subdir = (mime_dir_config *)subdirv;
- mime_dir_config *new =
- (mime_dir_config *)ap_palloc (p, sizeof(mime_dir_config));
-
- new-&gt;forced_types = ap_overlay_tables (p, subdir-&gt;forced_types,
- parent_dir-&gt;forced_types);
- new-&gt;encoding_types = ap_overlay_tables (p, subdir-&gt;encoding_types,
- parent_dir-&gt;encoding_types);
-
- return new;
-}
-</PRE>
-
-As a note --- if there is no per-directory merge function present, the
-server will just use the subdirectory's configuration info, and ignore
-the parent's. For some modules, that works just fine (<EM>e.g.</EM>, for the
-includes module, whose per-directory configuration information
-consists solely of the state of the <CODE>XBITHACK</CODE>), and for
-those modules, you can just not declare one, and leave the
-corresponding structure slot in the module itself <CODE>NULL</CODE>.<P>
-
-<H3><A NAME="commands">Command handling</A></H3>
-
-Now that we have these structures, we need to be able to figure out
-how to fill them. That involves processing the actual
-<CODE>AddType</CODE> and <CODE>AddEncoding</CODE> commands. To find
-commands, the server looks in the module's <CODE>command table</CODE>.
-That table contains information on how many arguments the commands
-take, and in what formats, where it is permitted, and so forth. That
-information is sufficient to allow the server to invoke most
-command-handling functions with pre-parsed arguments. Without further
-ado, let's look at the <CODE>AddType</CODE> command handler, which
-looks like this (the <CODE>AddEncoding</CODE> command looks basically
-the same, and won't be shown here):
-
-<PRE>
-char *add_type(cmd_parms *cmd, mime_dir_config *m, char *ct, char *ext)
-{
- if (*ext == '.') ++ext;
- ap_table_set (m-&gt;forced_types, ext, ct);
- return NULL;
-}
-</PRE>
-
-This command handler is unusually simple. As you can see, it takes
-four arguments, two of which are pre-parsed arguments, the third being
-the per-directory configuration structure for the module in question,
-and the fourth being a pointer to a <CODE>cmd_parms</CODE> structure.
-That structure contains a bunch of arguments which are frequently of
-use to some, but not all, commands, including a resource pool (from
-which memory can be allocated, and to which cleanups should be tied),
-and the (virtual) server being configured, from which the module's
-per-server configuration data can be obtained if required.<P>
-
-Another way in which this particular command handler is unusually
-simple is that there are no error conditions which it can encounter.
-If there were, it could return an error message instead of
-<CODE>NULL</CODE>; this causes an error to be printed out on the
-server's <CODE>stderr</CODE>, followed by a quick exit, if it is in
-the main config files; for a <CODE>.htaccess</CODE> file, the syntax
-error is logged in the server error log (along with an indication of
-where it came from), and the request is bounced with a server error
-response (HTTP error status, code 500). <P>
-
-The MIME module's command table has entries for these commands, which
-look like this:
-
-<PRE>
-command_rec mime_cmds[] = {
-{ "AddType", add_type, NULL, OR_FILEINFO, TAKE2,
- "a mime type followed by a file extension" },
-{ "AddEncoding", add_encoding, NULL, OR_FILEINFO, TAKE2,
- "an encoding (<EM>e.g.</EM>, gzip), followed by a file extension" },
-{ NULL }
-};
-</PRE>
-
-The entries in these tables are:
-
-<UL>
- <LI> The name of the command
- <LI> The function which handles it
- <LI> a <CODE>(void *)</CODE> pointer, which is passed in the
- <CODE>cmd_parms</CODE> structure to the command handler ---
- this is useful in case many similar commands are handled by the
- same function.
- <LI> A bit mask indicating where the command may appear. There are
- mask bits corresponding to each <CODE>AllowOverride</CODE>
- option, and an additional mask bit, <CODE>RSRC_CONF</CODE>,
- indicating that the command may appear in the server's own
- config files, but <EM>not</EM> in any <CODE>.htaccess</CODE>
- file.
- <LI> A flag indicating how many arguments the command handler wants
- pre-parsed, and how they should be passed in.
- <CODE>TAKE2</CODE> indicates two pre-parsed arguments. Other
- options are <CODE>TAKE1</CODE>, which indicates one pre-parsed
- argument, <CODE>FLAG</CODE>, which indicates that the argument
- should be <CODE>On</CODE> or <CODE>Off</CODE>, and is passed in
- as a boolean flag, <CODE>RAW_ARGS</CODE>, which causes the
- server to give the command the raw, unparsed arguments
- (everything but the command name itself). There is also
- <CODE>ITERATE</CODE>, which means that the handler looks the
- same as <CODE>TAKE1</CODE>, but that if multiple arguments are
- present, it should be called multiple times, and finally
- <CODE>ITERATE2</CODE>, which indicates that the command handler
- looks like a <CODE>TAKE2</CODE>, but if more arguments are
- present, then it should be called multiple times, holding the
- first argument constant.
- <LI> Finally, we have a string which describes the arguments that
- should be present. If the arguments in the actual config file
- are not as required, this string will be used to help give a
- more specific error message. (You can safely leave this
- <CODE>NULL</CODE>).
-</UL>
-
-Finally, having set this all up, we have to use it. This is
-ultimately done in the module's handlers, specifically for its
-file-typing handler, which looks more or less like this; note that the
-per-directory configuration structure is extracted from the
-<CODE>request_rec</CODE>'s per-directory configuration vector by using
-the <CODE>ap_get_module_config</CODE> function.
-
-<PRE>
-int find_ct(request_rec *r)
-{
- int i;
- char *fn = ap_pstrdup (r-&gt;pool, r-&gt;filename);
- mime_dir_config *conf = (mime_dir_config *)
- ap_get_module_config(r-&gt;per_dir_config, &amp;mime_module);
- char *type;
-
- if (S_ISDIR(r-&gt;finfo.st_mode)) {
- r-&gt;content_type = DIR_MAGIC_TYPE;
- return OK;
- }
-
- if((i=ap_rind(fn,'.')) &lt; 0) return DECLINED;
- ++i;
-
- if ((type = ap_table_get (conf-&gt;encoding_types, &amp;fn[i])))
- {
- r-&gt;content_encoding = type;
-
- /* go back to previous extension to try to use it as a type */
-
- fn[i-1] = '\0';
- if((i=ap_rind(fn,'.')) &lt; 0) return OK;
- ++i;
- }
-
- if ((type = ap_table_get (conf-&gt;forced_types, &amp;fn[i])))
- {
- r-&gt;content_type = type;
- }
-
- return OK;
-}
-
-</PRE>
-
-<H3><A NAME="servconf">Side notes --- per-server configuration, virtual
- servers, <EM>etc</EM>.</A></H3>
-
-The basic ideas behind per-server module configuration are basically
-the same as those for per-directory configuration; there is a creation
-function and a merge function, the latter being invoked where a
-virtual server has partially overridden the base server configuration,
-and a combined structure must be computed. (As with per-directory
-configuration, the default if no merge function is specified, and a
-module is configured in some virtual server, is that the base
-configuration is simply ignored). <P>
-
-The only substantial difference is that when a command needs to
-configure the per-server private module data, it needs to go to the
-<CODE>cmd_parms</CODE> data to get at it. Here's an example, from the
-alias module, which also indicates how a syntax error can be returned
-(note that the per-directory configuration argument to the command
-handler is declared as a dummy, since the module doesn't actually have
-per-directory config data):
-
-<PRE>
-char *add_redirect(cmd_parms *cmd, void *dummy, char *f, char *url)
-{
- server_rec *s = cmd-&gt;server;
- alias_server_conf *conf = (alias_server_conf *)
- ap_get_module_config(s-&gt;module_config,&amp;alias_module);
- alias_entry *new = ap_push_array (conf-&gt;redirects);
-
- if (!ap_is_url (url)) return "Redirect to non-URL";
-
- new-&gt;fake = f; new-&gt;real = url;
- return NULL;
-}
-</PRE>
-<!--#include virtual="footer.html" -->
-</BODY></HTML>